The invention relates to a rear projection screen having a front and a rear side for displaying at the front side of the screen a picture supplied by a plurality of primary picture sources arranged at the rear side of the screen, said screen comprising a translucent plate whose front side is provided with a multitude of parallel-arranged first light-spreading elements extending in one direction across the plate for spreading light in a plane perpendicular to the said direction, said elements having steep edges for obtaining total internal reflection and having a top with a portion for transmission and refraction. The invention also relates to a rear projection system comprising such a rear projection screen.
Rear projection screens are used in rear projection systems for video and cinematographic pictures and in various other types of display systems such as radar screens, airplane and ship simulators and microfilm readers. In such display systems a picture generated in a primary picture source is imaged by a system of projection lenses on the rear side of the rear projection screen. The projection screen spreads the incident light in the audience space at the front side of the screen.
Current projection systems for displaying color television or video programs comprise three primary picture sources for the primary colors red, green and blue which are projected on the screen by separate projection lens systems. The light intensity of each primary picture source is maximum on the optical axis of the relevant picture source and the associated projection lens system. Since the three picture sources the arranged beside or above each other, the said axes extend at an angle at the location of the projection screen so that without any further measures the viewer observes a color shift which depends on his position in the audience space.
U.S. Pat. No. 4,573,674 describes a rear projection screen in which this color shift in the horizontal direction is reduced to a minimum. This is achieved by providing vertically extending light-spreading elements at the front side of the screen whose ratio between height and width is at least 1:1 and whose top has a width which is at most half the width at the base, this top having a specially shaped indentation. In this respect the "height" of the element is understood to mean the maximum relief perpendicular to the plane of the screen. Light-absorbing material may be provided in the deep grooves between the elements so that reflection of ambient light is inhibited by the screen and the contrast in the displayed picture is enhanced. The light from the primary picture source incident on the rear side of the screen is guided to the tops of the profile due to total internal reflection on the steep edges so that there is a high transmission for this light. Due to the combination of steep edges and curved flatter tops a broad angular distribution in the horizontal direction is achieved. As a result of the repeated reflections on the edges the spread of light at the front side is largely independent of the position of the primary picture source.
The vertically extending elements spread the light entering the rear side in a horizontal direction in an angle of approximately 85.degree. to the left and right. In the vertical direction the light-spreading elements do not have any noticeable effect so that other measures are to be taken for spreading light in that direction. For example, roughening the rear side of the plate or providing the plate with a light-scattering material as proposed in U.S. Pat. No. 4,573,764. In the article "Ultrawide viewing angle rear projection screen" by R. Bradley et al., published in the magazine "IEEE Transactions on Consumer Electronics", vol. CE 31, no. 3, pp. 185-192 (1985) mention is made of the possibility of providing a so-called "bulk diffusor" in the plate, consisting of grains having a deviating refractive index.
It has been found that large-scale production of a screen as described in U.S. Pat. No. 4,573,764 meets with technical difficulties. Since the screen is manufactured through a replica process by means of a mould, the shape of the light-spreading elements must be introduced into the mould. This is done by means of a cutting tool having the required shape. For a screen having a diagonal dimension of approximately 100 cm, an aspect ratio of 3:4 and a pitch, i.e. the distance between the edge of an element to the same edge of the adjacent element, of 1 mm, the cutting tool is to cut along a distance of approximately 500 meters. For a screen in the proposed HDTV format with an aspect ratio of 9:16, also approximately 60 cm of picture height and a pitch of approximately 0.5 mm, the distance to be cut is approximately 1300 meters.
To prevent the light-spreading elements from changing in shape between the part of the mould which has been cut at an early stage and the part cut at a later stage, the cutting tool should substantially not be subject to wear. A change in shape is unacceptable because it affects the light spread which can clearly be observed by the viewer. For the same reason it is impossible to replace a worn cutting tool by a new one because then a small but clearly visible change in light spread occurs. For this reason only diamond is suitable as a material for the cutting tool. However, this presents the problem that it is not possible to give a diamond cutting tool the required concave shape as described in the said Patent.
Owing to these limitations a screen has to be accepted whose elements provided at the front side have a color spread which is less than optimum so that some color shift remains.